I would like to have a category field for the csv importer. I just searched through the bug reports and found that bug report 328140 marks this as fixed, but I can't see it.
4.6.6 is rather old at this point, almost three years. I didn't track down exactly which version the fix was in, but you should upgrade to 4.8.0, which was released almost a year ago.
Yep, that's what I did. I had to enable the kmymoney ppa for Kubuntu xenial for it. Although I use KDE Neon, it works well. So I mark this as fixed.